1. Field of the Invention
The present invention relates to a turbocharger system for a diesel engine having both exhaust gas recirculation (EGR) and water injection.
2. Disclosure Information
Modern diesel engines are equipped with exhaust gas recirculation (EGR) systems for reducing the amount of oxides of nitrogen (NOx) produced during combustion. In an EGR system, exhaust gases are routed to the intake of the engine. The presence of combusted gases in the cylinder inhibits NOx production by reducing the peak combustion temperatures. Heretofore, EGR was employed primarily during high torque operating conditions during which high quantities of fuel are combusted and high in-cylinder temperatures and pressures prevail, thus generating maximum levels of NOx. However, diesel engine designers are finding the use of EGR at low torque conditions, such as idle, enables meeting increasingly stringent emission regulations.
To obtain additional power output from a diesel engine of a given displacement, it is common to equip it with a turbocharger. Normally wasted exhaust enthalpy is recovered as work by an exhaust turbine. The shaft of the turbine is coupled to the shaft of a compressor disposed in the engine intake. The shaft work generated by the exhaust gases via the turbine is used to drive the air compressor to pressurize the intake gases.
Turbochargers frequently suffer from a problem known as turbo lag. That is, at low torque conditions, the flow through the engine, and hence the exhaust system, is low; thus, the rotational speed of the turbocharger is low. When the operator demands increased torque, the turbocharger is unable to speed up as quickly as desired to meet the torque demand. During such a transient, fueling is increased to the engine, which generates more power and higher enthalpy exhaust, thereby spinning up the turbine, which in turn compresses more air into the intake, allowing more fueling. Unfortunately, this process takes time, and as a result, engine torque output cannot be increased instantaneously. This problem has been overcome by system engineering encompassing low inertia turbochargers, turbochargers well matched to the engine, electronic control of fuel delivery, and other measures.
In an engine utilizing EGR at low torque conditions, the turbo lag problem is markedly exacerbated for two reasons: when a demand for an increase in torque is received, the fueling rate cannot be increased nearly as much as with no EGR because there is less fresh air in the combustion chamber. Moreover, depending upon the precise configuration employed, EGR may reduce the exhaust flow to the turbocharger, with the concomitant result that the turbocharger turns at a lower speed than if EGR weren""t employed. Consequently, vehicles with turbocharged engines using EGR frequently respond slowly to a demand for increased torque.
In general, it is known that water is an alternative to EGR for controlling NOx. The inventors of the present invention have recognized that if water is used for NOx control at low torque operating conditions, the turbocharger lag problem can be overcome. When using water, no exhaust gases are diverted into EGR; thus, the turbocharger rotates at a speed commensurate with a system not using EGR. Also, the amount of water required to obtain the desired NOx inhibition displaces less air than an amount of exhaust gases for the same NOx inhibition effect.
Diesel engine designers have used water injection in an effort to increase the power from turbocharged diesel engines. Typically, water injection is used during steady state operation to increase power output by cooling the combustion chamber with water. U.S. Pat. No. 4,558,665 discloses such an arrangement. Other engine designers have taught, however, that water injection will decrease the amount of fuel that can be burned, thereby decreasing power output. For example, U.S. Pat. No. 5,400,746 contains such a teaching. The inventors of the present invention propose to reduce so-called turbocharger lag through the use of water injection with a diesel engine during transient operation. Such operation will generally be triggered when the engine""s throttle pedal is opened at a rate in excess of a threshold value. Moreover, the present inventors teach that it is advantageous to cease injecting water as soon as the turbo lag is not perceptible, while increasing EGR flow rate, so as to decrease water consumption. In effect, additional oxygen enters the engine""s cylinders because the exhaust gases (EGR) which would be used according to prior art, are replaced with water, which occupies much less volume in the cylinder. Furthermore, the water vaporizes in the intake system and the intake temperature is decreased due to the latent enthalpy of vaporization of the water. Thus, the density of the air is increased.
Although a system and method according to the present invention maybe used with a diesel engine not having EGR, greater advantages may be realized with engines having electronic control of EGR flow.
A turbocharger system for a diesel engine includes an exhaust driven intake air compressor, a sensor for tracking the position of the engine""s throttle pedal and for generating a throttle position signal, and a water injection system for furnishing water to the engine""s air intake. An EGR valve furnishes recirculated exhaust gas to the engine""s air intake. Finally, a controller receives the throttle position signal and operates the water injection system and the EGR valve such that the rate of exhaust gas recirculation will be reduced and the rate of water injection increased in the event that the time rate of change of the throttle pedal position signal indicates that the throttle pedal is being depressed at a rate which exceeds a predetermined threshold value.
The added water may be injected into the engine""s air intake either upstream or downstream of the intake air turbocharger or other charge compressor. If the water is injected upstream of the turbocharger, the turbocharger output will greatly increase because the density of the air entering the turbocharger will be increased. Consequently, fuel injected into the engine can be increased without incurring an increase in soot or particulate emission. If, on the other hand, water is injected downstream of the turbo, the lag time before observing increased engine output will be minimized. In either event, the amount of fuel being injected into the engine will generally be increased whenever the rate of water injection is increased. This increased fuel charge will correspond with the increased availability of oxygen in the air charge entering the engine""s cylinders.
For an engine using both water injection and EGR according to the present invention, the amount of water being injected may be increased without reducing the flow rate, or amount, of EGR in the event that the EGR flow rate is below a predetermined threshold. On the other hand, if EGR flow rate is above a predetermined threshold, EGR rate will preferably be reduced as the amount of water being injected is increased. Water injection will be increased only until the turbocharger reaches an operating point sufficient to support the higher torque output desired by the driver or otherwise indicated by the depressed throttle pedal. In other words, when the turbocharger reaches a sufficiently high output point, injection of water into the engine will be cut off.
According to another aspect of the present invention, a method for controlling a turbocharged diesel engine includes the steps of tracking the position of the engine""s throttle pedal and generating a corresponding throttle pedal position signal, and injecting water into the engine""s air inlet in the event that the time rate of change of the throttle pedal position signal indicates that the throttle pedal is being opened at a rate exceeding a predetermined threshold. Finally, the amount of fuel being injected to the engine will be increased during the time corresponding to the injection of water. As described above, the injection of water will be increased until the turbocharger reaches an operating point sufficient to produce the torque indicated by the throttle operation and discontinued thereafter. At such time as water injection is decreased, EGR will be correspondingly increased.
It is an advantage of the present invention that a system and method according to the present invention will allow operation of a turbocharged diesel engine with lower or less objectionable turbo lag because the use of water injection during transient operation or tipin will allow increased oxygen to enter the engine""s cylinders, thereby allowing increased fueling without excessive exhaust emissions.
Other advantages as well as objects and features of the present invention will become apparent to the reader of this specification.